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Open-Circuit Voltage Improvement in Hybrid ZnO–Polymer Photovoltaic Devices With Oxide Engineering

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6 Author(s)
Lee, Yun-Ju ; Sandia Nat. Labs., Albuquerque, NM, USA ; Davis, R.J. ; Lloyd, M.T. ; Provencio, P.P.
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We present strategies to improve low open-circuit voltage (Voc) for ZnO-poly(3-hexylthiophene) (P3HT) photovoltaic devices, which are typically ≤0.4 V, but vary among different reports. One factor affecting Voc variability is the ZnO bandgap (Eg), which depends on detailed processing conditions. By decreasing the pyrolysis temperature of sol-gel ZnO films, we increased the ZnO Eg by 0.14 eV and Voc of corresponding bilayer devices by 0.1 V. This is understood as increased donor-acceptor energy-level offset. Next, we demonstrate significant enhancement in Voc by depositing conformal amorphous TiOx films at the surface of planar ZnO films and ZnO nanorod arrays using a spin-coating method. The TiOx coatings monotonically increased Voc from 0.4 to 0.8 V for devices with increasing TiOx thicknesses from 0 to ≥50 Å. Dark current-voltage measurement reveals that the TiOx coating significantly decreases the reverse-bias current density, leading to an improvement in Voc, in excellent agreement with predictions from the modified ideal diode equation. This is consistent with passivation of ZnO surface defects by TiOx. In short, by varying the solution processing conditions, we modify the bulk and interfacial properties of the metal oxide acceptor, thus leading to systematic improvement in open-circuit voltage.

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Selected Topics in Quantum Electronics, IEEE Journal of  (Volume:16 ,  Issue: 6 )